Method and apparatus for performing volume replication using unified architecture

ABSTRACT

Method and apparatus for performing volume replication using a unified architecture are provided. Each volume has an exclusive volume log table (VLT) and an exclusive volume block update table (VBUT). The VLT is mainly used for recording the relationship between two volumes of a mirroring pair, and the VBUT is used for tracking the state of each data block of the volume itself. By means of the cross operations and applications between the VLT and the VBUT, various volume replication processes such as volume copying and volume mirroring can be enabled under a unified architecture. For each volume, different replication relationships with other volumes can be handled merely by administering its two exclusive tables. The method and the apparatus provided by the present invention can simplify the architecture for synchronization replication and reduce the burdens of administrating tables, thereby making the operation of a storage system more efficient.

CROSS REFERENCE TO RELATED APPLICATION(S)

This application is a Divisional of U.S. patent application Ser. No.12/691,807, filed on Jan. 22, 2010, which claims the benefit ofprovisional Application No. 61/146,654, filed on Jan. 23, 2009, theentirety of which are incorporated by reference herein.

BACKGROUND

1. Field of Invention

The present invention relates to a method and an apparatus forperforming volume replication. More particularly, the present inventionrelates to a method and an apparatus for performing volume copying andvolume mirroring using a unified architecture.

2. Description of Related Art

In the modern societies, storage devices have been widely used invarious computer systems or storage systems. According to storagepurposes, some storage devices are used for storing the data needing tobe constantly accessed while a storage system is operated. Such storagedevices are referred to as primary storage devices. Another kind ofstorage devices is used for storing the backup data existing in theprimary storage device, and is referred to as a secondary storage devicewhich can replace the primary storage device to maintain normal systemoperations when the primary storage device fails or breaks down, therebyenhancing system reliability.

The storage devices can be hard disk drives, tapes, memory drives,optical storage drives or other storage devices for storing data. Suchstorage devices all are physical storage devices (PSDs).

A storage system (or computer system) does not actually consider onephysical storage device (PSD) as a unit when managing data, but combinesdifferent sections from one or more PSDs as a logical drive (LD) or as apartition. When a controller maps the LD or the partition to a host fordata access, the LD or the partition mapped is referred to as a logicalunit. A LD or partition can be further divided into one or more datastorage areas referred to as media extents. Many of data storage systemsmay replicate a portion or all of a volume to another volume integrally.Thus, whenever the original data are damaged or failed to be read, thereplicated data can be used to recover the original data or substitutedas the original data for other applications.

The source where the replicated data is copied from is referred to as asource volume, and the destination storing the replicated data isreferred to as a destination volume. The date replication with the unitof a volume can be used for the following purposes: backing up,reporting or archiving the source volume; or substituting the sourcevolume for performing any operations requiring the source volume, suchas simulating, predicting and data mining, etc. Especially, dataaccessing operations usually perform a locking mechanism on theto-be-accessed data before actually access it, and thus it is inevitableto affect data reading efficiency and also to affect the normal dataaccess of the host. Hence, if the destination volume is used tosubstitute the source volume for performing the aforementionedoperations, the source volume can provide the host data accessingservice without affecting the efficiency.

Volume copying and volume mirroring are two volume replication methodspopularly used in industries, in which the volume copying is used forcopying data of the source volume at a starting time point of thereplication operation (i.e. the time point of issuing a split command)and it is generally a one-time copy, and the volume mirroring is used toconsistently maintain a data synchronization relationship between twovolumes, i.e. considering the destination volume as a mirror of thesource volume. In conventional skills, the volume copying and the volumemirroring are implemented by two different architectures. Also, in theconventional skills, each of two volumes (i.e. source volume anddestination volume) of a mirror pair requires maintaining a trackingtable used for recording the mirroring state of each corresponding datablock on the volume (source volume or destination volume), and recordingthe information whether the corresponding data block has been modifiedduring the volume mirroring. A mirroring pair needs two tracking tablesrespectively stored in the source volume and the destination volume. Ifa volume A establishes a mirroring pair relationship respectively witheach of volumes B, C and D, then the volume A needs to administer threetracking tables. Due to the features of the aforementioned conventionalskills, the operations of the volume copying and the volume mirroringare quite complicated and inefficient.

Hence, if there is a unified architecture which can cover bothoperations of volume copying and volume mirroring and can simply theadministration of tracking tables, or in other words, if there is aunified architecture under which the volume copying and the volumemirroring are merely operational options of a single unified volumesynchronization function, then the operation efficiency of a storagesystem can be effectively promoted.

SUMMARY

Hence, an object of the present invention is to provide a method and anapparatus which can perform various volume replication operations suchas volume copying and volume mirroring by using a unified architecture.

According to an aspect of the present invention, a method for performingvolume replication is provided for allowing a destination volume to besynchronized (sync) with a source volume, and includes: allocating afirst entry in a volume log table (VLT) of the source volume, andinitializing a plurality of fields of the first entry comprising:setting a sequence number (SeqNum) defined in a first field of the firstentry to an initial setting value; and setting an operation code (OpCod)defined in a second field of the first entry to an operation coderepresenting a state of the source volume, wherein the initial settingvalue recorded in the first field of the first entry is further assignedto a first variable. The method for performing volume replicationfurther includes: allocating a second entry in a VLT of the destinationvolume, and initializing a plurality of fields of the second entrycomprising: setting an OpCod defined in a second field of the secondentry to an operation code representing a state of the destinationvolume. The method for performing volume replication further includes:adding an increment to a value of a master sequence number variable ofthe destination volume, and copying the value of the master sequencenumber variable of the destination volume into a first field of thesecond entry in the VLT of the destination volume as a SeqNum definedtherein, and into a fourth field of the first entry in the VLT of thesource volume as an alternative volume's sequence number (AltVolSN)defined therein, wherein the value of the master sequence numbervariable of the destination volume is further assigned to a secondvariable. The method for performing volume replication further includes:setting a fourth field of the second entry in the VLT of the destinationvolume as a starting address, wherein the fourth field of the secondentry is used for recording an index of a next data block to besynchronously replicated (DstVLT.NxtSynBlk); and performing a backgroundcopy (Background Copy of Source to Destination) procedure started fromthe starting address for copying the source volume to the destinationvolume.

According to another aspect of the present invention, a method forperforming volume replication is provided and applied in a process of asynchronization operation between a destination volume and a sourcevolume, wherein a split event occurs during the synchronizationoperation or when or after the synchronization operation is completed.The method for performing volume replication includes: allocating afirst entry in a volume log table (VLT) of the source volume, andinitializing a plurality of fields of the first entry comprising:setting a sequence number (SeqNum) defined in a first field of the firstentry to an initial setting value; and setting an operation code (OpCod)defined in a second field of the first entry to an operation coderepresenting a state of the source volume. The method for performingvolume replication further includes: allocating a second entry in a VLTof the destination volume, and initializing a plurality of fields of thesecond entry comprising: setting an OpCod defined in a second field ofthe second entry to an operation code representing a state of thedestination volume. The method for performing volume replication furtherincludes: adding an increment to a value of a master sequence numbervariable of the destination volume, and copying the value of the mastersequence number variable of the destination volume into a first field ofthe second entry in the VLT of the destination volume as a SeqNumdefined therein, and into a fourth field of the first entry in the VLTof the source volume as an alternative volume's sequence number(AltVolSN) defined therein, wherein the value of the master sequencenumber variable of the destination volume is further assigned to asecond variable. The method for performing volume replication furtherincludes: setting a fourth field of the second entry in the VLT of thedestination volume as a starting address, wherein the fourth field ofthe second entry is used for recording an index of a next data block tobe synchronously replicated (DstVLT.NxtSynBlk); and adding an incrementto a value of a master sequence number variable of the source volume,and copying the value of the master sequence number variable of thesource volume into a first field of the first entry in the VLT of thesource volume as a SeqNum defined therein, wherein the value of themaster sequence number variable of the source volume is further assignedto a first variable. The method for performing volume replicationfurther includes: allocating a third entry in the VLT of the destinationvolume, and initializing a plurality of fields of the third entrycomprising: setting an OpCod defined in a second field of the thirdentry to an operation code representing a split state. The method forperforming volume replication further includes: adding an increment tothe value of the master sequence number variable of the destinationvolume again, and copying the value of the master sequence numbervariable of the destination volume into a first field of the third entryin the VLT of the destination volume as a SeqNum defined therein; anddetermining if the synchronization operation between the source volumeand the destination volume is completed.

According to another aspect of the present invention, a disk arraycontroller able to implement a data replication operation performedamong a plurality of volumes in a physical storage device array (PSDarray) is provided and includes: a memory used for buffering datatransmitted between a host and the PSD array via the disk arraycontroller; and a central processing circuitry (CPC) using a volume logtable (VLT) and a volume block update table (VBUT) exclusively belongingto each of the volumes to perform the data replication operation,wherein an entry in the VLT has a first field used for recording asequence number (SeqNum), a second field used for recording an operationcode (OpCod), a third field used for recording an alternative volume'sidentification (AltVolID), and a fourth field used for recording asequence number (AltVolSN) of the entry in the VLT of the alternativevolume or for recording an index (NxtSynBlk) of a next data block itselfto be synchronized; and wherein an entry in the VBUT records updatedstate information of a data block.

According to another aspect of the present invention, a method forperforming volume replication operation includes forming a firstmirroring pair composed of a first volume and a second volume;administering a volume log table (VLT) and a volume block update table(VBUT) of the first volume, wherein the VLT is used for recordingsynchronization state information after the first volume and the secondvolume forms the first mirroring pair, and the VBUT is used forrecording updated state information of each data block of the firstvolume; forming a second mirroring pair composed of a first volume and athird volume; and administering the VLT and the VBUT of the firstvolume, wherein the VLT is used for recording synchronization stateinformation after the first volume and the third volume forms the secondmirroring pair, and the VBUT is used for recording updated stateinformation of each data block of the first volume.

It is to be understood that both the foregoing general description andthe following detailed description are examples, and are intended toprovide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood with regard to the followingdescription, appended claims, and accompanying drawings where:

FIG. 1 depicts a schematic diagram showing a hardware architecture of acomputer system applied in the present invention;

FIG. 2A depicts a data structure of a volume log table (VLT) provided bythe present invention;

FIG. 2B depicts a data structure of a volume block update table (VBUT)provided by the present invention;

FIG. 3 depicts a schematic diagram showing an embodiment of a storagevirtualization controller (SVC) used to implement the method of thepresent invention;

FIG. 4A to FIG. 4C depict a flow chart showing a method forsynchronously replicating a source volume to a destination volumeaccording to an embodiment of the present invention, wherein the sourcevolume and the destination volume are not split;

FIG. 5 depicts a flow chart showing a procedure of a volume block updatetable (VBUT) entry consistency check to the destination volume duringsynchronization (“Sync Process DstVBUT Entry Consistency Check”procedure) according to an embodiment of the present invention;

FIG. 6 depicts a flow chart showing a background copy procedure forcopying the source volume to the destination volume (“Background Copy ofSource to Destination” procedure) according to an embodiment of thepresent invention;

FIG. 7 depicts a flow chart showing a procedure of checking/copying datablocks from the source volume to the destination volume (“Check/CopyBlock from Source to Destination Volume” procedure) according to anembodiment of the present invention;

FIG. 8 depicts a flow chart showing a procedure of checking whether datablocks of the source volume and the destination volume are insynchronization (“Check Volume Block In-Sync” procedure) according to anembodiment of the present invention;

FIG. 9A to FIG. 9D depict a flow chart showing a method forsynchronously replicating a source volume to a destination volumeaccording to an embodiment of the present invention, wherein the sourcevolume and the destination volume have been split, and the time point ofsplitting the two volumes may occur during synchronization, whensynchronization is completed, or after synchronization is completed;

FIG. 10A to FIG. 10E depict actual examples of the volume log tables(VLTs) at a time point T according to the present invention;

FIG. 11A to FIG. 11D depict actual examples of the volume block updatetables (VBUTs) at a time point T according to the present invention,which are respectively corresponding to the examples shown in FIG. 10Ato FIG. 10D;

FIG. 12A to FIG. 12E depict actual examples of the volume log tables(VLTs) at a time point T+t according to the present invention; and

FIG. 13A to FIG. 13D depict actual examples of the volume block updatetables (VBUTs) at a time point T+t according to the present invention,which are respectively corresponding to the examples shown in FIG. 12Ato FIG. 12D.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings. Wherever possible, the same reference numbers areused in the drawings and the description to refer to the same or likeparts.

The present invention is mainly to provide a method which uses a unifiedarchitecture to implement various volume replications including “volumecopying” and “volume mirroring”. Whenever a volume involves a mirroringpair with its alternative volume, its exclusive volume log table (VLT)and volume block update table (VBUT) are established. Thereafter, nomatter how many synchronization or mirroring relationships are furtherestablished between the volume and other volumes, the volume merely hasto administer one exclusive VLT and one exclusive VBUT. Thus, thetechnology provided by the present invention can simplify thecomplicated replication architecture used in the conventional technologyand reduce the quantity of tables to be administered advantageously, andenables a storage system to be operated more efficiently.

In the method of performing volume replication, a “mirroring pair”relationship is first established between a source volume and adestination volume, and then, an initial mirroring procedure is startedto replicate the source volume to the destination volume, which isreferred to “synchronization” or “sync”. During a sync procedure, when ahost issues a write request to the source volume, if the block to bewritten has not been copied to the destination volume associated withthe source volume yet, the written data carried by the write request hasmerely to be written into the source volume; otherwise, the written datahas to be written into both the associated data blocks in the sourcevolume and the destination volume of the same mirroring pair. After theinitial sync procedure (or called initial mirroring procedure) iscompleted, both of the source and destination volumes of the mirroringpair enter a synchronization state, and thus any data changes of thesource volume thereafter have to be simultaneously replicated to thedestination volume. At this stage, the destination volume is consideredas a mirror of the source volume. In accordance with the applicationneeds, the mirroring relationship between the two volumes of themirroring pair can be stopped at a certain time point, and suchoperation is referred to as a “split” operation. When a split command isissued, it means that the destination volume has to reserve the state ofthe source volume at the time point at which the split command isissued, and it also means that, after the time point, the mirroring-pairrelationship between the source volume and the destination volume isbroken, and the two volumes can be independently accessed or updatedwithout needing to maintain the consistence between their respectivedata changes.

The split operation may occur at any time points during the mirroringprocess, which is mirroring the source volume to the destination volume,or after the completion of the mirroring process, or even, the splitoperation may not occur all the time. One of the methods to performingthe split operation is issuing the split command as soon as themirroring operation is started. This method, referred to as “volumecopying”, intends to replicate the state of the source volume at thetime point at which the split command is issued, and in this situationthe split command is generally a one-shot command. Relatively, if themirroring relationship is substantially maintained between the twovolumes all the time, and no split events occur during thesynchronization, at or after the completion of the synchronization, thatis, the synchronization relationship constantly existed between the twovolumes, this replication method is referred to as “volume mirroring”.

Referring to FIG. 1, it is a schematic diagram showing a hardwarearchitecture of a computer system 1 applied in the present invention.The computer system 1 includes a host 11 and a storage virtualizationsubsystem (SVS) 12 connected thereto. Although only one host 11 and onlyone SVS 12 are shown in this embodiment, yet in actual applications, onehost 11 may be connected to a plurality of SVSs 12; a plurality of hosts11 may be connected to one SVS 12; or a plurality of hosts 11 may beconnected to a plurality of SVSs 12.

The host 11 can be a host computer, such as a server system, aworkstation, a personal computer, etc. The SVS 12 includes a storagevirtualization controller (SVC) 13 and a physical storage device array(PSD array) 14 connected thereto. The SVC 13 can be a disk arraycontroller or a JBOD (Just a Bunch Of Disks) emulator. Although only onePSD array 14 connected to one SVC 13 are shown in the figure, yet inactual applications, one or more PSD arrays 14 can be used and the host11 also can be a SVC.

The SVC 13 receives an IO (Input/Output) request and its carried data(including control and data signals) from the host 11, and performs theIO request to map the IO request onto the PSD array 14 for performingdata access, and also replies related data back to the host 11 if any.The PSD array 14 includes a plurality of PSDs. The PSDs can be, forexample, hard disks. The SVC 13 can be used for enhancing performanceand/or improving data availability, or used for increasing the storagecapacity of one single logical media unit viewed by the host 11.

A volume is a virtualized logical storage unit. The SVC 13 virtualizesthe PSD array 14 as one or more logical storage devices and presents thelogical storage devices to the host 11 for accessing data. The host 11can determine how many volumes should be formed from the one or morelogical storage devices; each volume has its own file system. One volumemay cover one or more PSDs, partial areas of the PSDs, or a partial areaof only one PSD. For convenient expression in FIG. 1, it is assumed thatthe storage area of one volume just covers one PSD, but otherembodiments of the present invention have no the limitation. Just asdescribed above, one volume in the present invention may cover a partialarea of one PSD, one or more PSDs, or the partial areas of the PSDs.

When the computer system 1 is performing a volume replication operation,all the contents of the entire source volume are completely copied tothe destination volume. For example, as shown in FIG. 1, it is likely tocopy a volume A 15 a wholly to a volume B 15 b, or maybe other possibleexamples. In this example, the volume A 15 a acting as a data provideris a source volume, while the volume B 15 b receiving the replicateddata is a destination volume. The both capacities of the source volumeand the destination volume may not be the same, but in general, thecapacity of the destination volume has to be greater than or equal tothe one of the source volume, thereby accommodating all of the datareplicated from the source volume.

The capacity of one volume may be from a couple of gigabytes to hundredsof gigabytes, even up to terabytes. It takes some time to completelyreplicate (or called “sync” or “mirror”) such large amount of data fromone volume to the other volume. After the completion of the datareplication, if no split command is issued, the “sync” state will bemaintained between the two volumes. While the “mirroring” or “sync”state exists between the two volumes, the host 11 still can access datafrom the source volume. Further, the source volume or the destinationvolume after being split from the source volume may establish any kindsof volume replication relationships with other volumes individually. Forrecording the related information, referring to FIG. 2A and FIG. 2B,according to an embodiment of the present invention, whenever a volumeis generated, a volume log table (VLT) 30 exclusively belonging to thevolume itself is established for recording the sync state informationbetween the volume itself and each of the other volumes, and a volumeblock update table (VBUT) 40 exclusively belonging to the volume itselfis established for recording the updated state information of each datablock of the volume itself. According another embodiment of the presentinvention, whenever a volume involves a mirroring pair with anothervolume, an exclusive VLT 30 s is established for recording the syncstate information between the two volumes, and an exclusive VBUT 40 isestablished for recording the updated state information of each datablock of the volume itself.

FIG. 2A merely depicts one entry and the fields contained therein in theVLT 30 as a representation for explanation. However, in actualoperations, a VLT 30 may contain one or more VLT entries. Whenever theowner of the VLT 30 establishes a new relationship with another volumeor has a state change itself, a new VLT entry is accordingly generated.One VLT entry contains four fields, which are a first field 31 forrecording a sequence number (SeqNum), a second field 32 for recording anoperation code (OpCod), a third field 33 for recording an alternativevolume identification (AltVolID) in which the alternative volume isassociated with the owner of this VLT 30 (VLT owner), and a fourth field34 possessed of two possible functions in which one is for recording analternative volume sequence number (AltVolSN), which is the sequencenumber of an entry in the VLT of the alternative volume associated withthis VLT entry, and the other is for recording a next-sync-block index(NxtSynBlk) of the VLT owner itself, which is the next data block to besynchronized. The sequence of the four fields in a VLT entry shown inthe above is merely one embodiment of the present invention, and inother embodiments, the four fields can be arranged in other sequences,which should fall within the scope or spirit of the present invention aslong as the aforementioned information of the four fields can becompletely recorded. The details of the four fields are furtherexplained hereinafter.

At the beginning of creating a VLT entry, the sequence number (SeqNum)recorded in the first field 31 is an initial setting value. Taking thesource volume as an example for explanation, the initial setting valuecan be “0 (null)” if, meaning that the source volume owning the VLT 30has not been split off from the destination volume of which theidentification (AltVolID) is recorded in the third field 33. Thereafter,if a split operation occurs, the sequence number (SeqNum) is assigned bya master sequence number (MstSN) variable. The master sequence numbervariable is incremented from “1”, if also taking “0 (null)” being theinitial setting value of the sequence number (SeqNum) as an example.That is, the sequence number (SeqNum) of a VLT entry first entering thesplit state is “1”, the second sequence number (SeqNum) of another VLTentry entering the split state thereafter is “2”, and so forth. In sum,as long as the value of the first field 31 of the VLT entry is not “0(null)”, it means that the source volume owning the VLT 30 has beensplit off from the destination volume of which the identification(AltVolID) is recorded in the third field 33, and on the contrary, ifthe value of the first field 31 of the VLT entry is “0 (null)”, it meansthat the two volumes has not been split yet.

If the VLT 30 is belong to a destination volume, the initial settingvalue of the sequence number (SeqNum) recorded in the first field 31 ofthe VLT entry can be “1”, representing the first sync event occurring.The sequence number (SeqNum) of the VLT entry associated with anothersync event occurring later is also assigned by a master sequence number(MstSN) variable, which can be incremented from “2”. As to whether thedestination volume has been split off from the source volume, it can bedetermined by the second field 32 of the VLT entry in the destinationvolume, and has nothing to do with the sequence number (SeqNum) recordedin the first field 31 thereof.

Certainly, in other embodiments, the initial setting value of thesequence number (SeqNum) recorded in the first field 31 of the VLT entryof the source volume and that of the destination volume also can be setto other numeric values, and further, the incremental offset of themaster sequence number (MstSN) also can be another numeric value insteadof “1”. For example, the initial setting value can be set to “2”, andthe incremental offset of the master sequence number (MstSN) variablecan be set to “2”, and thus the master sequence number (MstSN) variablewill assign the values based on the pattern of 2, 4, 6 and so on to thesequence numbers (SeqNums) sequentially appearing in the first field 31of the VLT entry. It should be noted that, if the initial setting valueset to the sequence number (SeqNum) recorded in the first field 31 ofthe VLT entry of the source volume is not “0 (null)”, it is based onwhether the sequence number (SeqNum) is the initial setting value or notto determine whether the source volume has entered a split state or not.For example, if the initial setting value is “1”, a sequence number(SeqNum) of “1” means that the source volume has not been split off fromthe destination volume, and then the sequence number (SeqNum) of the VLTentry is assigned by the initial setting value plus an offset when thesource volume is split.

In the present invention, there are three definitions of the operationcode (OpCod) recorded in the second field 32 of the VLT entry, meaningthat a volume can be situated at one of three states listed as follows:

SYNSRC: representing that the volume acts as a source volume in asynchronization relationship of two volumes;

SYNDST: representing that the volume acts as a destination volume in asynchronization relationship of two volumes; and

SYNSPL: representing that the destination volume has been split off froma source volume.

The aforementioned three kinds of operation codes (OpCods) also mayaffect the contents recorded in the fourth field 34 of the VLT 30. Ifthe operation code (OpCod) recorded in the second field 32 is “SYNSRC”or “SYNSPL”, the fourth field 34 in the same VLT entry records thealternative volume sequence number (AltVolSN), which is the sequencenumber of an entry in the VLT of the alternative volume associated withthis VLT entry; and if the operation code (OpCod) recorded in the secondfield 32 is “SYNDST”, then the fourth field 34 in the same VLT entryrecords a next-sync-block index (NxtSynBlk) of the VLT owner itself,which is the next data block to be synchronized.

It should be noted that, although the above paragraphs use “SYNSRC”,“SYNDST” and “SYNSPL” to represent three states of a volume as being a“source volume”, a “destination volume” and having been “split” at aspecific moment, yet the present invention is not limited thereto. Inother embodiments, other codes can be used to represent these threestates.

FIG. 2B merely depicts one entry containing fields in the VBUT 40 as arepresentation for explanation. However, in actual operations, a VBUT 40may contain a plurality of VBUT entries, and each VBUT entry representsa data block of its owner; that is, the amount of the VBUT entries in aVBUT belonging to a volume is equal to the amount of the data blocks ofthe volume. One VBUT entry may contain two fields, which are a firstfield 41 representing a reserved field, and a second field 42 used forrecording the sequence number of its associated VLT entry, meaning thatthe data in the data block associated with the VBUT entry are updateddue to the event represented by the sequence number of the VLT entry,thereby tracking the state of each data block of the volume owning theVBUT, the state such as whether the data block has been synchronized orwhether the data block is changed again after synchronization due to thedata access from the host 11. The data length of one VBUT entry may be32 bits or another proper length. In other embodiments, one VBUT entrymay contain other number of fields in addition to two, and may onlycontain a second field 42 if simplification is considered.

A replication method provided by the present invention for performingvolume copying and volume mirroring under a unified architecture isenabled by the cross operations and applications between the VLT 30 andthe VBUT 40, which will be described in detail later. The apparatusapplying the method of the present invention is the SVC 13 shown in FIG.1, and a schematic diagram showing the structure of the SVC 13 accordingto one embodiment is as shown in FIG. 3.

The main function of the SVC 3 is to map various sections of PSDs toform one or more logical storage devices visible to the host 11. Afterbeing received by the SVC 13, a host IO request issued from the host 11first will be parsed and interpreted, and the related operations anddata will be translated into one or more PSD IO requests.

In this embodiment, the SVC 13 includes a host-side IO deviceinterconnect controller 131, a central processing circuit (CPC) 132, amemory 133 and a device-side IO device interconnect controller 134.Although these components are described with separate functional blocksherein, yet in actual applications, parts or all of the functionalblocks can be integrated into one single chip.

The host-side IO device interconnect controller 131, which is connectedto the host 11 and the CPC 132 as an interface and a buffer between theSVC 13 and the host 11, can receive an IO request and related data sentfrom the host 11, and coverts and transmits the IO request and therelated data to the CPC 132.

The memory 133 is connected to the CPC 132 as a bufferer for bufferingthe data transmitted between the host 11 and the PSD array 14 throughthe CPC 132. In an actual application, the memory 133 can be a dynamicrandom access memory (DRAM), and the DRAM can be a synchronous dynamicrandom access memory (SDRAM).

The device-side IO device interconnect controller 134, which is disposedbetween the CPC 132 and the PSD array 14 as an interface and a bufferbetween the SVC 13 and the PSD array 14, can receive the IO request andthe related data sent from the CPC 132, and maps and/or transmits the IOrequest and the related data to the PSD array 14.

The CPC 132 is a calculation center of the SVC 13. When the SVC 13receives the host IO request from the host-side IO device interconnectcontroller 131, the CPC 132 will parse the host IO request, and performsome operations to respond to the host IO request, and transmit therequested data and/or reports and/or information from the SVC 13 to thehost 11 via the host-side IO device interconnect controller 131. Thevolume replication method of the present invention can be enabled byprogramming codes. The program codes are stored in an internal memory(such as a ROM (not shown)) of the CPC 132 or the external memory 133,so that the program codes can be executed by the CPC 132.

There are some principles needing to be observed for performing thevolume replication method of the present invention, which are explainedas follows.

The First Principle

Any volume which is involved a sync operation cannot be designated as adestination volume of another sync operation. In other words, if avolume, which is participating one sync operation, is required to be thedestination volume of another sync operation, the performing syncoperation has be stopped first and the volume has to be split off fromthe original mirroring pair, and afterward the volume can be designatedas the destination volume of another sync operation.

The Second Principle

An unsplit destination volume which is participating a sync operationcannot be presented alone to the host for data access. Only the sourcevolume can be the accessed target to the host. Due to suchcharacteristic, if a destination volume is temporarily off-line, it canreturn to be on-line to continuously perform the sync operation with theoriginal sync configuration. Based on the same reason, since adestination volume cannot be a single target for the host IO request, itcannot be a source volume to another sync operations unless it has beensplit.

The Third Principle

During a sync operation and before the split of the source anddestination volumes, if a re-synchronization (resync) operation isactivated between the two volumes, the two volumes have to be splitpromptly via the steps of: setting the sequence number of the VLT entryof the source volume to an up-to-date sequence number, and adding a“SYNSPL entry” to the VLT of the destination volume, and then the resyncoperation can be performed.

The Fourth Principle

During a sync operation and before the split of the source anddestination volumes, if the destination volume is off-line and meanwhilethe source volume has updated its data due to a host IO request and theupdated data block(s) has(have) been synchronously replicated, the twovolumes have to be split promptly via the following step: setting thesequence number of the VLT entry of the source volume is an up-to-datesequence number. When the destination volume returns to be on-line, thesequence number (SeqNum) field 31 of the VLT entry of the source volumewill be checked. If the field 31 is not the initial setting value suchas “0 (null)”, the destination volume will be recorded as “split” viathe following step: adding a “SYNSPL entry” to the VLT of thedestination volume. Thereafter, the synchronization operation can bere-activated to resynchronize the data in the two volumes.

The Fifth Principle

During a sync operation and after the time point that the two volumesare split off from each other, if the source volume has data changes butthe original data in the updated data blocks cannot be copied to thedestination volume beforehand, an “error bit” will be set on the latest“SYNSRC entry”. In this situation, if the current sync operation has notfinished yet, any actions try to perform a resync operation will beprohibited until the current sync operation is completed. Theaforementioned step of setting an error bit on the VLT entry of thesource volume is merely one embodiment of the present invention, and inother embodiments, the error bit can be set on the VLT entry of thedestination volume, or stored in another recording table independently.

Before the detailed description of the processes of the method of thepresent invention, the definitions of some specific code names areexplained as follows.

DstMstSN: a master sequence number variable of a destination volume;

SrcMstSN: a master sequence number variable of a source volume;

DstVBUT: a VBUT entry corresponding to a data block of a destinationvolume which is being processed;

SrcVBUT: a VBUT entry corresponding to a data block of a source volumewhich is being processed;

DstVLT: a VLT entry of a destination volume which is in operation; and

SrcVLT: a VLT entry of a source volume which is in operation.

Referring to FIG. 4A to FIG. 4C, which depict a flow chart showing amethod of synchronously replicating a source volume to a destinationvolume according to an embodiment of the present invention, wherein thesource volume and the destination volume are not split off from eachother yet, and a relationship of sync or mirroring is being maintainedbetween the two volumes. Please refer to FIG. 2A and FIG. 2Bsimultaneously for the below description. In FIG. 4A, when the processbegins, it is first performed to determine if the procedure is aninitial sync operation (step 201). If the determination result of step201 is yes, the VBUT 40 of the destination volume is initialized (step202) and the DstMstSN variable of the destination volume is initialized(step 203). The way of initialization is to set each entry in the VBUT40 and the DstMstSN variable to be equal to their initial setting valuesrespectively. The initial setting values can be “0 (null)” in oneembodiment or another possible values in the other embodiments.

If the determination result of step 201 is not a initial sync operationor after steps 202 and 203 are performed, step 204 is performed toallocate an entry (SrcVLT) in the VLT 30 of the source volume andinitialize the fields thereof appropriately, including setting the OpCodof the second field 32 to “SYNSRC”, and setting the sequence number ofthe first field 31 to an initial setting value such as “0 (null)”.Further, the initial setting value of the sequence number is assigned toa first variable which is referred to as a “SRCCURSPL” variable in thisembodiment. Then, step 205 is performed to allocate an entry (DstVLT) inthe VLT 40 of the destination volume and initialize the fields thereofappropriately, including setting the OpCod of the second field 32 to“SYNDST”. Then, step 206 is performed to add one unit to the value ofthe DstMstSN variable and copy the value of the DstMstSN variable to thefirst field 31 of the DstVLT (shown as DstVLT.SeqNum) and to the fourthfield 34 of the SrcVLT (shown as SrcVLT.AltVolSN). Further, the currentvalue of the DstMstSN variable is assigned to a second variable which isreferred to as a “DSTCURSYN” variable. Then, step 207 is performed toset the fourth field 34 of the DstVLT (shown as DstVLT.NxtSynBlk) to theblock index 0, standing for a data block of which the LBA (logical blockaddressing) index is 0.

After completion of the above steps, there are two procedures followingand executed in parallel, one of which is a procedure of “BackgroundCopy of Source to Destination” shown in step 208 (the details of theprocedure as shown in FIG. 6) and the other one of which is a procedureactivated by the appearance of a host IO request by entering the joinpoint A 211 (the details of the procedure as shown in FIG. 4B and FIG.4C as described later).

The “Background Copy of Source to Destination” procedure called by step208 is mainly performing an operation of copying data from the sourcevolume to the destination volume in a background environment. During theoperation of this procedure, a determination condition is consistentlychecked, which is whether all of the data copy processes are completed(step 209). If the determination result is yes, then the entireprocedure of FIG. 4A goes to the end; otherwise, the “Background Copy ofSource to Destination” procedure is continuously running. The details ofthe “Background Copy of Source to Destination” procedure will bedescribed when FIG. 6 is introduced.

As described above, it takes a certain period of time to completely copythe source volume with an enormous capacity to the destination volume.During this period of time, the host 11 still need to access data fromthe source volume at any time as usual. It should be noted that, sincethe destination volume is not split off from the source volume yet, thesecond principle described above has to be followed, which is, anunsplit destination volume which is participating a sync operation cannot be presented alone to the host for data access. Hence, in thisprocedure, only the source volume can be presented as a data accesstarget for the host 11.

Referring to FIG. 4B, after entering the present process flow from thejoin point A 211, step 212 is first performed to inquire if there is anyhost IO request targeting the source volume. If the determination resultof step 212 is no, the present process flow stays at the inquiry stateof step 212; if the determination result of step 212 is yes, the presentprocess flow enters step 213 to parse the host IO request and transformit into one or more LD (logical drive) IO (Input/Output) requests, eachof which is targeting a data block of the source volume to be accessed.Thereafter, the process flow is divided into two directions: one goes toreturn to step 212 to continuously determine if there is any host IOrequest targeting the source volume; the other goes to execute theprocess flow of FIG. 4C requested by each of the LD IO requests (step214). For example, if there are n number of LD IO requests (LD IO₁ . . .LD IO_(n)) going to access n number of data blocks in the source volume,there will be n number of process flows (steps 214-1 . . . 214-n) ofFIG. 4C triggered and executed simultaneously. Whenever the process flowassociated with any one of the LD IO requests is completed, step 215 isperformed to determine whether all of the process flows (steps 214-1 . .. 214-n) executed in parallel are completed or not. If the determinationresult of step 215 is no, the process flow stays at the inquiry state ofstep 215; if the determination result of step 215 is yes, step 216 isperformed to response to the host that the host IO request is completed,and then the process flow returns to step 212 to determine if there isany other host IO request targeting the source volume.

It should be noted that, in FIG. 4B, as long as a host IO requestappears to access data from the source volume, the host IO request isimmediately processed by entering the process flow via step 212, nomatter whether the previous host IO request(s) is(are) completed or not.

Referring to FIG. 4C, it shows a procedure for accessing a data blockassociated with each of the LD IO requests. The procedure shown in FIG.4C is equivalent to a processing block of step 214 shown in FIG. 4B. Atfirst, step 220 is performed to determine if the host IO request isintending to write data into the source volume; that is, if the host IOrequest is a volume write request. If the determination result of step220 is no, it means that the host IO request is intending to read data,so step 221 is performed for directly reading data from a data block inthe source volume (source volume block), and then the procedure returnsto step 215 shown in FIG. 4B for checking the determination conditionthereof. If the determination result of step 220 is yes, step 222 isperformed to further determine if the destination volume is accessible.If the destination volume is not accessible at the time, a“synchronization error flag (SyncErrorFlag)” is set to “TRUE” (step223), and the data carried by the host IO request (Host IO data) whichis to be written is only copied to the source volume block (step 230),and then the procedure returns to step 215 shown in FIG. 4B for checkingthe determination condition thereof. If the determination result of step222 is yes, step 224 is performed to call a procedure of “Sync ProcessDstVBUT Entry Consistency Check”, which performs the consistency checkof the VBUT entry of the destination volume, and to check if thereturned value of the “Sync Process DstVBUT Entry Consistency Check”procedure is “FALSE” or not. Wherein the details of the “Sync ProcessDstVBUT Entry Consistency Check” procedure is shown in FIG. 5 and willbe explained later. If the determination result of step 224 is yes, afailure message is asserted and issued (step 225). If the determinationresult of step 224 is no or after step 225 is completed, step 226 isperformed to further determine if the index of the data block of thesource volume being modified (going to be written) is less than theDstVLT.NxtSynBlk, which is the index of the data block next to besynchronously replicated and is recorded in the fourth field 34 of theVLT entry of the destination volume. If the determination result of step226 is yes, it means that the data block of the source volume to bewritten has been synchronously replicated to the destination volume, andthus the data (Host IO data) to be written by the host IO request has tobe copied into the corresponding data block on the destination volume(step 227), thereby maintaining the two volumes in the synchronizationstate. Afterward, step 228 is performed to further determine if theDstVBUT.SeqNum is equal to the value of the DSTCURSYN variable (thesecond variable); namely, if the sequence number of the VBUT entry ofthe destination volume is equal to the value of the DSTCURSYN variable.If the determination result of step 228 is no, the DstVBUT.SeqNum is setto be equal to the value of the DSTCURSYN variable (the second variable)(step 229), meaning that the corresponding data block of the destinationvolume is updated during the event represented by the DSTCURSYNvariable. If the determination result of step 228 is yes or after step229 is completed, step 230 is performed to copy the host IO data (thedata to be written by the host IO request) to the source volume block(the associated data block in the source volume). If the determinationresult of step 226 is no, step 230 is performed as well to copy the hostIO data to the source volume block. Thereafter, the procedure shown inFIG. 4C goes to the end and returns to step 215 shown in FIG. 4B forchecking the determination condition thereof.

Referring to FIG. 5, it depicts a flow chart showing a procedure of aVBUT entry consistency check to the destination volume duringsynchronization (“Sync Process DstVBUT Entry Consistency Check”procedure) according to an embodiment of the present invention. Thisprocedure is called by step 224 shown in FIG. 4C, and mainly has threedetermination conditions. The procedure returns a message of “FALSE”(step 305) as long as any of the three determination conditions is met,and returns a message of “TRUE” if all of the three determinationconditions are not met (step 304). The three determination conditionsare described in detail as follows.

The determination condition 1 performed at step 301 is to determine ifthe DstVBUT.SeqNum is greater than the value of the DSTCURSYN variable(the second variable); namely, if the sequence number of the VBUT entry,which is currently processed, of the destination volume is greater thanthe value of the DSTCURSYN variable. Since the destination volume is notsplit off from the source volume yet when the procedure shown in FIG. 5is executed, the determination condition 1 that the DstVBUT.SeqNum isgreater than the value of the DSTCURSYN variable should not happen.Therefore, in case that the determination condition 1 is met, thisprocedure returns a message of “FALSE” immediately (step 305);otherwise, this procedure goes to check the next determinationcondition.

The determination condition 2 performed at step 302 is to determine ifthe DstVBUT.SeqNum is equal to the value of the DSTCURSYN variable (thesecond variable) (namely, if the sequence number of the VBUT entry,which is currently processed, of the destination volume is equal to thevalue of the DSTCURSYN variable), and if the DstVLT.NxtSynBlk, standingfor the index of the next data block to be synchronously replicated(NxtSynBlk) recorded in the fourth field 34 of the VLT entry, which iscurrently processed, of the destination volume, is less than or equal tothe index of the data block which is being modified. Since the tworespective conditions of the determination condition 2 are contradictoryto each other, this procedure will return a message of “FALSE”immediately (step 305) when the determination condition 2 is met;otherwise, this procedure goes to check the next determinationcondition.

The determination condition 3 performed at step 303 is to determine ifthe DstVLT.NxtSynBlk, which stands for the index of the next data blockto be synchronously replicated (NxtSynBlk) recorded in the fourth field34 of the VLT entry of the destination volume which is currentlyprocessed, is greater than the index of the data block which is beingmodified (meaning that the data block has be synchronously replicated),and if a procedure of checking whether data blocks of the source volumeand the destination volume are in synchronization (“Check Volume BlockIn-Sync” procedure) returns a value of “FALSE”. Wherein the details ofthe “Check Volume Block In-Sync” procedure will be shown in FIG. 8 anddescribed later. Because the data blocks, which have been synchronouslyreplicated, in the destination volume have to keep their states untilthe destination volume is split off from the source volume, thisprocedure will return a message of “FALSE” when the determinationcondition 3 is met (step 305); otherwise, the procedure returns amessage of “TRUE” (step 304).

Referring to FIG. 8, it depicts a flow chart showing a procedure ofchecking whether data blocks of the source volume and the destinationvolume are in synchronization (“Check Volume Block In-Sync” procedure)according to an embodiment of the present invention. Wherein the “CheckVolume Block In-Sync” procedure is called by step 303 of FIG. 5described above, and also will be called by other procedures describedlater. This procedure manly has five determination conditions. Theprocedure returns a message of “FALSE” (step 607) as long as any of thefive determination conditions is met, and returns a message of “TRUE” ifall of the five determination conditions are not met (step 606). Thefive determination conditions are described in detail as follows.

The determination condition 1 performed at step 601 is to determine ifthe DstVBUT.SeqNum (standing for the sequence number of the VBUT entry,which is currently processed, of the destination volume) is equal to theinitial setting value, such as “0 (Null)”. If the determination resultof step 601 is yes, it means that the data block is still under thestage of the initialization operation and certainly is not at “In-Sync(already in synchronization)” state. Hence, a message of “FALSE” isreturned (step 607); otherwise, this procedure goes to check the nextdetermination condition.

The determination condition 2 performed at step 602 is to determine ifthe VLT entry of the destination volume associated with theDstVBUT.SeqNum (standing for the sequence number of the VBUT entry,which is currently processed, of the destination volume) is missing, orcan be found but the data cannot be read (maybe corrupted), or the datais inconsistent. If the determination result of step 602 is yes, amessage of “FALSE” is returned (step 607); otherwise, this proceduregoes to check the next determination condition.

The determination condition 3 performed at step 603 is to determine ifthe DstVLT.OpCod (standing for the OpCod recorded in the second field 32of the VLT entry, which is currently processed, of the destinationvolume) is not “SYNDST”, or the DstVLT.AltVolID (standing for theAltVolID recorded in the third field 33 of the VLT entry, which iscurrently processed, of the destination volume) is not the volumeidentification (ID) of the source volume corresponding thereto. If thedetermination result of step 603 is yes, a message of “FALSE” isreturned (step 607); otherwise, this procedure goes to check the nextdetermination condition.

The determination condition 4 performed at step 604 is to determine ifthe VLT entry of the source volume with the SrcVLT.AltVolSN (standingfor the sequence number (AltVolSN) of the VLT entry of the destinationvolume recorded in the fourth field 34 of the VLT entry of the sourcevolume), which is equal to the DstVBUT.SeqNum (standing for the sequencenumber of the VBUT entry, which is currently processed, of thedestination volume), is missing, or can be found but cannot be read(maybe corrupted), or is inconsistent. If the determination result ofstep 604 is yes, a message of “FALSE” is returned (step 607); otherwise,this procedure goes to check the next determination condition.

The determination condition 5 performed at step 605 is to determine ifthe SrcVBUT.SeqNum (standing for the sequence number of the VBUT entry,which is currently processed, of the source volume) is greater than orequal to the SrcVLT.SeqNum (standing for the sequence number (SeqNum)recorded in the first field 31 of the VLT entry, which is currentlyprocessed, of the source volume). If the determination result of step605 is yes, it means that the data in the data block on the sourcevolume are changed again after being synchronously replicated to thedestination volume, so a message of “FALSE” is returned (step 607);otherwise, a message of “TRUE” is returned (step 606).

Referring to FIG. 6, it depicts a flow chart showing the procedure ofBackground Copy of Source to Destination according to an embodiment ofthe present invention. This procedure is called by step 208 shown inFIG. 4A described above, and also is called by other proceduresdescribed later. For each data block represented by a VBUT entry of thedestination volume (step 401), step 402 is performed to call and executea procedure of checking/copying data blocks from the source volume tothe destination volume (“Check/Copy Block from Source to Destination”procedure), wherein the details of the “Check/Copy Block from Source toDestination” procedure is shown in FIG. 7 and will be described later.After step 402 is completed, step 403 is performed to determine if thereare still other data blocks of the source volume left to besynchronously replicated. If the determination result of step 403 isyes, the block number of the DstVLT.NxtSynBlk (standing for the index ofthe next data block to be synchronously replicated (NxtSynBlk) recordedin the fourth field 34 of the VLT entry, which is currently processed,of the destination volume) is incremented by 1 unit (step 404), and thenthe procedure returns to step 401 to perform another round of steps 402to 403 for the next data block. If the determination result of step 403is no, the value of the DstVLT.NxtSynBlk (standing for the index of thenext data block to be synchronously replicated (NxtSynBlk) recorded inthe fourth field 34 of the VLT entry, which is being processed, of thedestination volume) is set to “−1” (step 405), meaning that all of thedata blocks have been synchronously replicated, and then this proceduregoes to the end. It should be noted that setting the value of theDstVLT.NxtSynBlk to “−1” is merely an embodiment of the presentinvention; in other embodiments, any expression using a proper numericalvalue to achieve the same effect should fall within the scope or spiritof the present invention, for example, setting the DstVLT.NxtSynBlk to“−2”.

Referring to FIG. 7, it depicts a flow chart showing a procedure ofchecking/copying data blocks from the source volume to the destinationvolume (“Check/Copy Block from Source to Destination” procedure)according to an embodiment of the present invention, wherein theprocedure is called by step 402 shown in FIG. 6. At first, step 501 isperformed to determine if the DstVBUT.SeqNum (standing for the sequencenumber of the VBUT entry, which is currently processes, of thedestination volume) is less than the value of the DSTCURSYN variable(the second variable). If the determination result of step 501 is yes,step 502 is performed to further determine if the destination volume issplit off from the source volume and if the SrcVBUT.SeqNum (standing forthe sequence number of the corresponding VBUT entry of the sourcevolume) is greater than or equal to the value of the SRCCURSPL variable(the first variable). If the determination result of step 502 is yes, itmeans that the data block of the source volume has been modified and itsoriginal data before the modification is not copied to the destinationvolume beforehand, thus causing the data in the corresponding data blockof the destination volume to be unsynchronized with the one in thesource volume, so that the VBUT entry of the corresponding data block ofthe destination volume is marked as “DATA INVALID” (step 503), and thenthe procedure goes to the end. If the determination result of step 502is no, step 504 is performed to execute the procedure of “Check VolumeBlock In-Sync”, the details of which are already shown and explained inFIG. 8 above, and determine if its returned value is “FALSE”. If thedetermination result of step 504 is yes, it means that the correspondingdata blocks of the source volume and the destination volume are notsynchronized yet, and thus step 505 is performed to copy the block dataof the source volume to the destination volume. Then, step 506 isperformed to set the DstVBUT.SeqNum (standing for the sequence number ofthe VBUT entry, which is currently processes, of the destination volume)to the value of the DSTCURSYN variable (the second variable), andthereafter the procedure goes to the end. If the determination result ofstep 504 is no, it means that the corresponding data blocks of thesource volume and the destination volume have been synchronized, andthus no further process is required and the procedure directly goes tothe end.

If the determination result of step 501 is no, step 507 is performed tofurther determine if the destination volume is split off from the sourcevolume. If the determination result of step 507 is yes, step 508 isperformed to skip the block and the procedure goes to the end. If thedetermination result of step 507 is no, step 509 is performed to asserta failure message, and then the procedure goes to the end.

Referring to FIG. 9A to FIG. 9D, the four figures together depict a flowchart showing a method for synchronously replicating a source volume toa destination volume according to another embodiment of the presentinvention, wherein the source volume and the destination volume havebeen split off from each other, and the split time point may beappearing during the synchronization, when or after the synchronizationis completed. The process flow shown in FIG. 9A to FIG. 9D may beperformed after steps 201-207 shown in FIG. 4A, meaning that a splitevent occurs at a specific time point during or after thesynchronization of the source volume and the destination volume. Pleasealso refer to FIG. 2A and FIG. 2B for the process flow described below.At first, the value of the SrcMstSN is incremented by 1 unit and copiedto the first field 31 of the VLT of the source volume (SrcVLT.SeqNum);in addition, the current value of the SrcMstSN is assigned to theaforementioned first variable (SRCCURSPL) (step 701). Then, step 702 isperform to allocate an entry in the VLT 30 of the destination volume(DstVLT) and initialize its fields appropriately including setting theoperation code (OpCod) recorded in the second field 32 to “SYNSPL”.Afterward, the value of the DstMstSN variable is incremented by 1 unitand copied into the first field 31 of the DstVLT (DstVLT.SeqNum), andadditionally, the current value of the DstMstSN is assigned to a thirdvariable (referred to as “DSTCURSPL” in this embodiment) (step 703). Itshould be noted that before entering step 703 to be incremented by 1unit, the value of the DstMstSN variable has been assigned to the secondvariable (DSTCURSYN) in step 206 shown in FIG. 4A, and thus the value ofthe third variable (DSTCURSPL) is different from that of the secondvariable (DSTCURSYN) by 1 unit. After the completion of the above steps,there are two procedures executed in parallel subsequently, including:the procedure of “Background Copy of Source to Destination” (itsdetailed process flow is already shown and explained in FIG. 6 above)called by step 705, and a procedure after entering a join point B 706(its detailed process flow will be shown in FIG. 9B to FIG. 9D and willbe explained later), which is activated by the appearance of a host IOrequest.

The procedure of “Background Copy of Source to Destination” called bystep 705 mainly executes an operation for copying the data of the sourcevolume to the destination volume in a background environment. During theoperation of the procedure of “Background Copy of Source toDestination”, step 704 is consistently performed to determine if thesynchronization operation of the source volume to the destination volumeis completed. If the determination result of step 704 is yes, theprocess flow for the background copy goes to the end; otherwise, theprocedure of “Background Copy of Source to Destination” is continuouslyperformed until the determination result of step 704 is yes and then theprocess flow for the background copy goes to the end.

Referring to FIG. 9B, after entering the procedure from the join point B706, step 707 is performed to inquire if there is any host IO requesttargeting the source volume or the destination volume. It is noted that:since, in the present process flow, the destination volume has beensplit off from the source volume, the two volumes can respectively bethe target of the host IO request. If the determination result of step707 is no, it is stayed at the inquiry state of step 707. If thedetermination result of step 707 is yes, step 708 is performed to parsethe host IO request and separate it into one or more LD IO requests,each of which is targeting an associated data block of the source volumeor the destination volume to be accessed. Thereafter, the process flowis divided into two directions: one goes to return to step 707 tocontinuously determine if there is any host IO request targeting thesource volume or the destination volume; the other goes tosimultaneously execute the process flows of FIG. 9C and FIG. 9Drequested by each of the LD IO requests (step 709). For example, ifthere are n number of LD IO requests (LD IO₁ . . . LD IO_(n)) going toaccess n number of data blocks in the source volume or the destinationvolume, there will be n number of process flows (steps 709-1 . . .709-n) of FIG. 9C and FIG. 9D triggered and executed simultaneously.Whenever the process flow associated with any one of the LD IO requestsis completed, step 711 is performed to determine whether all of theprocess flows (steps 709-1 . . . 709-n) executed in parallel arecompleted or not. If the determination result of step 711 is no, theprocess flow returns to a join point D 710 and stays at the inquirystate of step 711. If the determination result of step 711 is yes, step712 is performed to response to the host that the host IO request iscompleted, and then the process flow returns to step 707 to continuouslydetermine if there is any other host IO request targeting the sourcevolume or the destination volume.

It should be noted that, in FIG. 9B, as long as a host IO requestappears to access data from the source volume or the destination volume,the host IO request is immediately processed by entering the processflow via step 707, no matter whether the previous host IO request(s)is(are) completed or not.

Referring to FIG. 9C and FIG. 9D, both together show a procedure foraccessing a data block associated with each of the LD IO requests. Theprocedure shown in FIG. 9C and FIG. 9D is equivalent to a processingblock of step 709 shown in FIG. 9B. At first, step 720 is performed todetermine if the host IO request is intending to write data into thesource volume or the destination volume. If the determination result ofstep 720 is no, it means that the host IO request is intending to readdata, so the procedure enters the steps shown in FIG. 9D by a join pointC 732 to perform related process flow of reading data. The relatedprocess flow shown in FIG. 9D will be described later. If thedetermination result of step 720 is yes, it means that the host IOrequest is intending to write data, so step 721 is performed to furtherdetermine if the destination volume is accessible. If the destinationvolume cannot be accessed at the time, a “synchronization error flag(SyncErrorFlag)” is set to “TRUE” (step 726). Then, step 727 isperformed to further determine if the Host IO request is targeting thesource volume for writing data thereto. If the determination result ofstep 727 is yes, the data to be written by the host IO request (Host IOdata) are copied to the corresponding data block on the source volume(step 728). Going back to step 721, if the determination result of step721 is yes, it means that the destination volume is accessible at thetime, and then step 722 is performed to determine if the host IO requestis targeting the source volume for writing data thereto. If thedetermination result of step 722 is yes, step 723 is performed to calland execute the procedure of “Check/Copy Block from Source toDestination”, thereby checking and determining if it is necessary tocopy the original data of the data block, which is currently processed,of the source volume to the corresponding data block of the destinationvolume beforehand. The details of the “Check/Copy Block from Source toDestination” procedure are already shown and described in FIG. 7 in theabove. If the determination result of step 722 is no, step 724 isperformed to further determine if the host IO request is targeting thedestination volume for writing data and only a part of the data blockneeds to be modified. If the determination result of step 724 is yes,the process flow goes to step 723 as well. If the determination resultof step 724 is no or after step 723 is completed, step 725 is performedto copy the data to be written by the host IO request (Host IO data) tothe targeting data block in the source volume or the destination volume.After step 725 and step 728 are completed, step 729 is performed tofurther determine if the host IO request is targeting the source volumefor writing data. If the determination result of step 729 is no, step730 is performed to set the DstVBUT.SeqNum equal to the value of thethird variable (DSTCURSPL), meaning that the data in the modified datablock of the destination volume are updated in the event represented bythe value of SRCCURSPL. If the determination result of step 729 is yes,step 731 is performed to set the SrcVBUT.SeqNum equal to the value ofthe first variable (DSTCURSPL), meaning that the data in the modifieddata block of the source volume are updated in the event represented bythe value of SRCCURSPL. Thereafter, the process flow shown in FIG. 9Cgoes to the end and returns to FIG. 9B via the join point D 710, andthen step 711 is entered to perform the determination.

If the determination result of step 720 is no, the process flow goes toFIG. 9D via the join point C 732 to read data. At first, step 733 isperformed to determine if the host IO request is targeting thedestination volume for reading data. If the determination result of step733 is yes, step 734 is performed to further determine if the VBUT entrycorresponding to the data block to be read is marked as “DATA INVALID”.If the determination result of step 734 is yes, an error message isreturned (step 736), and then the procedure of FIG. 9D goes to the endand returns to FIG. 9B via the join point D 710. If the determinationresult of step 734 is no, step 735 is performed to further determine ifthe synchronization error flag (SyncErrorFlag) of the destination volumeis ever set to “TRUE”, and the DstVBUT.SeqNum (standing for the sequencenumber of the VBUT entry, which is currently processed, of thedestination volume) is less than the value of the second variable(DSTCURSYN), and the SrcVBUT.SeqNum (standing for the sequence number ofthe VBUT entry, which is currently processed, of the source volume) isgreater than or equal to the first variable (SRCCURSPL). If thedetermination result of step 735 is yes, step 737 is performed to markthe VBUT entry corresponding to the data block to be written as “DATAINVALID”. Then the process flow goes to step 736 as well to return anerror message, and then the procedure of FIG. 9D goes to the end andreturns to FIG. 9B via the join point D 710. If the determination resultof step 735 is no, step 738 is performed to determine if the host IOrequest is targeting the source volume for reading data. If thedetermination result of step 738 is yes, step 740 is performed to readdata from the data block in the source volume, and the procedure returnsto FIG. 9B via the join point D 710. If the determination result of step738 is no, step 739 is performed to further determine if the host IOrequest is targeting the destination volume for reading data and theDstVBUT.SeqNum (standing for the sequence number of the VBUT entry,which is currently processed, of the destination volume) is less thanthe value of the second variable (DSTCURSYN). If the determinationresult of step 739 is yes, it means that the data in the data block tobe read is not synchronized with the corresponding data block in thesource volume yet. Thus, the process flow turns to step 740 to read datafrom the corresponding data block in the source volume. If thedetermination result of step 739 is no, step 741 is performed todirectly read data from the data block in the destination volume. Thenthe process flow goes to the end and returns to FIG. 9B via the joinpoint D 710.

An actual example is taken below to explain how to achieve variousvolume replication methods, including volume copying and volumemirroring, using a unified architecture by administering the VLT and theVBUT in the present invention.

Referring to FIG. 10A and FIG. 11A, both are schematic diagramsrespectively showing the VLT and the VBUT of a volume A (VolA) at a timepoint T. As shown in FIG. 10A, it can be seen from its second field 32and third field 33 that, at time point T, the VolA is the source volumeof each of volumes B, C and E (VolB, VolC and VolE, respectively).Moreover, as shown in FIG. 10A, it can be seen from the sequence numbers(SeqNum) recorded in the first field 31 that the VolA has been split offfrom the volume B (VolB) and the volume C (VolC) (since their sequencenumbers are not “0”) but is not split off from the volume E (VolE) yet(since its sequence number is “0”). Moreover, as shown in FIG. 10A, itcan be seen from the fourth field 34 that the corresponding sequencenumbers of the volume A (VolA) stored in the VLTs of the volumes B, Cand E (VolB, VolC, VolC) all are “1”. As shown in FIG. 11A, the topmostrow and the leftmost column are the indexes of the respective VBUTentries expressed in hexadecimal, and FIG. 11B to FIG. 11D, and FIG. 13Ato FIG. 13D described below also have the same index expression. In FIG.11A, with reference to FIG. 10 A, the VBUT entries with the sequencenumber of “1” (such as those with the VBUT entry indexes of 0x01, 0x12,0x25 and 0x3A) indicates that the data blocks corresponding thereto arethe data blocks which have data changes due the access of the host IOrequest after the volume A (VolA) and the volume B (VolB) are spilt offfrom each other. Similarly, in FIG. 11A, the VBUT entries with thesequence number of “2” (such as those with the VBUT entry indexes of0x04, 0x16, 0x28, 0x2E and 0x31) indicate that the data blockscorresponding thereto are the data blocks which have data changes duethe access of the host IO request(s) after the volume A (VolA) and thevolume C (VolC) are spilt off from each other. Other VBUT entries with asequence number of “0” indicate the data blocks corresponding theretoare the ones which have no data changes until the time point T since thevolume A (VolA) has the first split operation. It is noted that “0x”used in this disclosure stands for a hexadecimal value.

Referring to FIG. 10 B and FIG. 11B, both are schematic diagramsrespectively showing the VLT and the VBUT of the volume B (VolB) at thetime point T. It can be seen from FIG. 10 B that, at the time point T,the volume B (VolB) is the destination volume of the volume A (VolA) andhas been split off therefrom, and meanwhile, the volume B (VolB) is thesource volume of the volume D (VolD) but not split off yet. The value of“−1” shown in the fourth field 34 of the first VLT entry in FIG. 10Brepresents that the synchronization replication operation between thevolume B (VolB) and the volume A (VolA) has been completed; the “SYNSPL”shown in the second field (OpCod) 32 of the second VLT entry representsthat the volume B (VolB) has been split off from the volume A (VolA). Ifa volume acts as the destination volume, the operation codes (OpCod)stored in its second fields 32 often have “SYNDST” and “SYNSPL” shown ina pair. In FIG. 11B, with reference to FIG. 10B, the VBUT entries withthe sequence number of “1” indicate that the data blocks correspondingthereto are the data blocks which have been synchronized after theestablishment of the synchronization relationship between the volume B(VolB) and the volume A (VolA). Further, the VBUT entries with thesequence number of “2” indicate that the data blocks correspondingthereto are the data blocks which have data changes due the access ofthe host IO request(s) after the volume B (VolB) is split off from thevolume A (VolA).

Referring to FIG. 10C and FIG. 11C, both are schematic diagramsrespectively showing a VLT and a VBUT of the volume C (VolC) at the timepoint T. It can be seen from FIG. 10C that, at the time point T, thevolume C (VolC) is the destination volume of the volume A (VolA), andthe two volumes have been split off from each other but do not completethe synchronization replication operation yet. In FIG. 10C, the “C”shown in the fourth field 34 of the first VLT entry indicates“NxtSynBlk” representing that the index of the next data block of thevolume C (VolC) to be synchronously replicated is “0x0C”. Since thevalue stored in this field (NxtSynBlk) is not “−1”, it can be seen thatthe volume C (VolC) has not been completely synchronized with the volumeA (VolA) yet. The value of “2” shown in the fourth field 34 of thesecond VLT entry indicates “AltVolSN” representing that the volume C(VolC) has been split off from the volume A (VolA), and thecorresponding sequence number of the VLT entry of the volume A (VolA) is“2”. In FIG. 11C, with reference to FIG. 10C, the VBUT entries with thesequence number of “1” indicate that the data blocks correspondingthereto are the data blocks which have been synchronized after theestablishment of the synchronization relationship between the volume C(VolC) and the volume A (VolA). Further, the VBUT entries with thesequence number of “2” indicate that the data blocks correspondingthereto are the data blocks which have data changes due the access ofthe host Io request(s) after the volume C (VolC) is split off from thevolume A (VolA). Since the initial value of the sequence number of eachVBUT entry is set to “0” by the initialization procedure and the volumeC (VolC) merely acts as the destination volume of the volume A (VolA) atthe time point T, the VBUT entries with the sequence number of “0”indicate that the corresponding data blocks in the volume C (VolC) havenot been synchronized yet or have no data changes due to any host IOrequest.

Referring to FIG. 10D and FIG. 11D, both are schematic diagramsrespectively showing the VLT and the VBUT of the volume D (VolD) at thetime point T. There is only one VLT entry shown in FIG. 10D. It can beseen from the entry that the volume D (VolD) is the destination volumeof the volume B (VolB), and the two volumes are not split off from eachother yet, and the index of the next data block of the volume D (VolD)to be synchronized is “0x19”. Referring to FIG. 11D, it is obvious thatthe sequence numbers of the VBUT entries before the index of “0x19” areall equal to “1”, and those thereafter are all equal to “0”, meaningthat the volume D (VolD) is only involved with the synchronousreplication operation with the volume B (VolB). Since the volume D(VolD) is not split off from the volume B (VolB) yet, the host cannotsee the volume D (VolD), so that there is no host IO request targetingthe volume D (VolD) for data access.

Referring to FIG. 10E, it is a schematic diagram showing a VLT of thevolume E (VolE) at the time point T. There is only one VLT entry shownin FIG. 10E. It can be seen from the entry that the volume E (VolE) isthe destination volume of the volume A (VolA), and the two volumes arenot split off from each other yet, and the index of the next data blockof the volume E (VolE) to be synchronized is “0x####”.

From the summarization of the information expressed in FIG. 10A to FIG.10E, it can be seen that, at the time point T, the volume A (VolA) isthe source volume of each of the volumes B, C and E (VolB, VolC, VolE),and the volume B is the source volume of the volume D (VolD), and

1. the volume B (VolB) and the volume A (VolA) have been split off fromeach other and have completed the synchronization replication operationtherebetween;

2. the volume C (VolC) and the volume A (VolA) have been split from eachother but have not completed the synchronization replication operationyet;

3. the volume D (VolD) and the volume B (VolB) have not been split fromeach other and also have not completed the synchronization replicationoperation yet; and

4. the volume E (VolE) and the volume A (VolA) have not been split fromeach other and also have not completed the synchronization replicationoperation yet.

After a period of time t, that is, at a time point T+t, the respectiveVLTs of the volumes A, B, C, D and E (VolA, VolB, VolC, VolD, VolE) areas shown in FIG. 12A to FIG. 12E, and the respective VBUTs of thevolumes A, B, C and D (VolA, VolB, VolC, VolD) are as shown in FIG. 13Ato FIG. 13D. These figures, similar to FIG. 10A to FIG. 10E and FIG. 11Ato FIG. 11D, record the information in accordance with theaforementioned principles and methods. The detailed contents of therespective figures can be interpreted similarly by following thedescription of FIG. 10A to FIG. 10E and FIG. 11A to FIG. 11D, and thusare not described in detail herein again. In sum, FIG. 12A to FIG. 12Eand FIG. 13A to FIG. 13D represent the following points.

1. A resync operation is activated between the volume A (VolA) and thevolume B (VolB), and the two volumes have not split off from each otherand have not completed the synchronization operation yet;

2. the volume D (VolD) and the volume B (VolB) have split off from eachother and have completed the synchronization operation;

3. the state between the volume C (VolC) and the volume A (VolA) stillstays at “split but the synchronization operation not completed yet”;however, the “NxtSynBlk” has progressed from “0x0C” to “0x27”; and

4. the state between the volume E (VolE) and the volume A (VolA) stillstays at “unsplit and the synchronization operation not completed yet”.

It can be seen from the above description that each volume provided bythe present invention has its exclusive VLT and VBUT, and with the helpof the cross operations between the VLT and the VBUT, variousreplication methods including volume copying and volume mirroring can beenabled under a unified architecture. Moreover, for each volume, varioustypes and many numbers of replication relationships with other volumescan be handled merely by administering its two exclusive tables of VLTand VBUT. Hence, the present invention can advantageously simplify thearchitecture for synchronization replication operation and reduce theburdens of administrating tables, thereby making the operation of astorage system more efficient.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the structure of the presentinvention without departing from the scope or spirit of the invention.In view of the foregoing, it is intended that the present inventioncover modifications and variations of this invention provided they fallwithin the scope of the following claims and their equivalents.

What is claimed is:
 1. A method for performing volume replicationoperation, comprising: forming a first mirroring pair composed of afirst volume and a second volume; providing at least one first volumelog table (VLT) and a first volume block update table (VBUT) for thefirst volume, and providing at least one second VLT and a second VBUTfor the second volume; administering the at least one first VLT and thefirst VBUT and the at least one second VLT and the second VBUT, whereinthe at least one first VLT and the at least one second VLT are used forrecording synchronization state information after the first volume andthe second volume forms the first mirroring pair for a firstsynchronization operation, and the first VBUT and second VBUT are usedfor recording updated state information of each data block of the firstvolume and the second volume, respectively; forming a second mirroringpair composed of a first volume and a third volume; providing at leastone third VLT and a third VBUT for the third volume; and administeringthe at least one first VLT and the first VBUT and the at least one thirdVLT and the third VBUT, wherein the at least one first VLT and the atleast one third VLT are used for recording synchronization stateinformation after the first volume and the third volume forms the secondmirroring pair for a second synchronization operation, and the firstVBUT and the third VBUT are used for recording updated state informationof each data block of the first volume and the third volume,respectively; wherein the first VBUT, the second VBUT, and the thirdVBUT have a plurality of entries each of them used for recording asequence number to represent the updated state information of each datablock of the first volume, the second volume, and the third volume,respectively.
 2. The method of claim 1, wherein one of two replicationoperations composed of volume copying and volume mirroring is performedbetween the first volume and the second volume of the first mirroringpair.
 3. The method of claim 2, wherein one of two replicationoperations composed of volume copying and volume mirroring is performedbetween the first volume and the third volume of the second mirroringpair.
 4. The method of claim 1, wherein the first VBUT, administered bythe first volume, is used for recording updated state information ofeach data block of the first volume when the first mirroring paircomposed of the first volume and the second volume is formed, and thesame first VBUT, also administered by the first volume, is used forrecording updated state information of each data block of the firstvolume when the second mirroring pair composed of the first volume andthe third volume is formed.
 5. The method of claim 4, wherein the atleast one first VLT comprises a first-to-second VLT, administered by thefirst volume, for recording synchronization state information of thefirst mirroring pair composed of the first volume and the second volume.6. The method of claim 5, wherein the at least one first VLT comprises afirst-to-third VLT, administered by the first volume, for recordingsynchronization state information of the second mirroring pair composedof the first volume and the third volume.
 7. The method of claim 5,wherein the first-to-second VLT is used for recording synchronizationstate information of the second mirroring pair composed of the firstvolume and the third volume.
 8. The method of claim 5, furthercomprising: allocating a first entry in the first-to-second VLT when thefirst mirroring pair composed of a first volume and a second volume isformed; and initializing a plurality of fields of the first entrycomprising: setting a sequence number (SeqNum) defined in a first fieldof the first entry to an first initial setting value, setting anoperation code (OpCod) defined in a second field of the first entry toan operation code (OpCod) representing a state of the first volume, andsetting an alternative volume sequence number (AltVolSN) or a next syncblock index (NxtSynBlk) defined in a forth field of the first entry,wherein the alternative volume sequence number (AltVolSN) is onerepresenting an entry in one of the at least one second VLT and thefirst next sync block index (NxtSynBlk) is one representing a next datablock, starting from a starting address, to be synchronized of the firstvolume.
 9. The method of claim 8, further comprising: setting a thirdfield of the first entry of the first-to-second VLT to represent theidentity of the second volume.
 10. The method of claim 8, furthercomprising: providing a first-to-third VLT in the at least one firstVLT, said first-to-third VLT being administered by the first volume, forrecording synchronization state information of the second mirroring paircomposed of the first volume and the third volume; allocating a firstentry in the first-to-third VLT of the first volume when the secondmirroring pair composed of a first volume and a third volume is formed;and initializing a plurality of fields of the first entry comprising:setting a sequence number (SeqNum) defined in a first field of the firstentry to an first initial setting value, setting an operation code(OpCod) defined in a second field of the first entry to an operationcode (OpCod) representing a state of the first volume, and setting analternative volume sequence number (AltVolSN) or a next sync block index(NxtSynBlk) defined in a forth field of the first entry, wherein thealternative volume sequence number (AltVolSN) is one representing anentry in one of the at least one third VLT and the first next sync blockindex (NxtSynBlk) is one representing a next data block, starting from astarting address, to be synchronized of the first volume.
 11. The methodof claim 10, further comprising: setting a third field of the firstentry of the first-to-third VLT to represent the identity of the thirdvolume.
 12. The method of claim 1, wherein the first volume and thesecond volume of the first mirroring pair are respectively a sourcevolume and a destination volume in the first mirroring pair.
 13. Themethod of claim 12, further comprising: allocating a first entry in theat least one first VLT of the source volume, and initializing aplurality of fields of the first entry comprising: setting a sequencenumber (SeqNum) defined in a first field of the first entry to aninitial setting value; and setting an operation code (OpCod) defined ina second field of the first entry to an operation code representing astate of the source volume; wherein the initial setting value recordedin the first field of the first entry is further assigned to a firstvariable; allocating a second entry in the at least one second VLT ofthe destination volume, and initializing a plurality of fields of thesecond entry comprising: setting an OpCod defined in a second field ofthe second entry to an operation code representing a state of thedestination volume; adding an increment to a value of a master sequencenumber variable of the destination volume, and copying the value of themaster sequence number variable of the destination volume into a firstfield of the second entry in the at least one second VLT of thedestination volume as a SeqNum defined therein, and into a fourth fieldof the first entry in the at least one first VLT of the source volume asan alternative volume's sequence number (AltVolSN) defined therein,wherein the value of the master sequence number variable of thedestination volume is further assigned to a second variable; setting afourth field of the second entry in the at least one second VLT of thedestination volume as a starting address, wherein the fourth field ofthe second entry is used for recording an index of a next data block tobe synchronously replicated (DstVLT.NxtSynBlk); and performing abackground copy (Background Copy of Source to Destination) procedurestarted from the starting address for copying the source volume to thedestination volume.
 14. The method of claim 12, wherein a split eventoccurs during the synchronization operation or when or after thesynchronization operation is completed, the method comprising:allocating a first entry in the at least one first VLT of the sourcevolume, and initializing a plurality of fields of the first entrycomprising: setting a sequence number (SeqNum) defined in a first fieldof the first entry to an initial setting value; and setting an operationcode (OpCod) defined in a second field of the first entry to anoperation code representing a state of the source volume; allocating asecond entry in the at least one second VLT of the destination volume,and initializing a plurality of fields of the second entry comprising:setting an OpCod defined in a second field of the second entry to anoperation code representing a state of the destination volume; adding anincrement to a value of a master sequence number variable of thedestination volume, and copying the value of the master sequence numbervariable of the destination volume into a first field of the secondentry in the at least one second VLT of the destination volume as aSeqNum defined therein, and into a fourth field of the first entry inthe at least one first VLT of the source volume as an alternativevolume's sequence number (AltVolSN) defined therein, wherein the valueof the master sequence number variable of the destination volume isfurther assigned to a second variable; setting a fourth field of thesecond entry in the at least one second VLT of the destination volume asa starting address, wherein the fourth field of the second entry is usedfor recording an index of a next data block to be synchronouslyreplicated (DstVLT.NxtSynBlk); adding an increment to a value of amaster sequence number variable of the source volume, and copying thevalue of the master sequence number variable of the source volume into afirst field of the first entry in the at least one first VLT of thesource volume as a SeqNum defined therein, wherein the value of themaster sequence number variable of the source volume is further assignedto a first variable; allocating a third entry in the at least one secondVLT of the destination volume, and initializing a plurality of fields ofthe third entry comprising: setting an OpCod defined in a second fieldof the third entry to an operation code representing a split state;adding an increment to the value of the master sequence number variableof the destination volume again, and copying the value of the mastersequence number variable of the destination volume into a first field ofthe third entry in the at least one second VLT of the destination volumeas a SeqNum defined therein; and determining if the synchronizationoperation between the source volume and the destination volume iscompleted.
 15. The method of claim 14, further comprising: performing abackground copy procedure for copying the source volume to thedestination volume.
 16. The method of claim 13, further comprising:determining if there is a host IO (Input/Output) request targeting thesource volume or the destination volume for data access.
 17. The methodof 1, further comprising: performing a first background copy procedurefor copying data blocks of the first volume to the second volume in thefirst synchronization operation; detecting whether a host IO requestfrom a host is targeting the first volume; and processing one or morelogical drive IO requests associated the host IO when a host IO requesttargeting the first volume is detected.
 18. The method of 17, whereinthe step of processing one or more logical drive IO requests furthercomprising: when the host IO request is a write request targeting thefirst volume and it is determined that data stored in the data blocks ofthe first volume that are associated with the host IO request have beencopied to the second volume due to the first background copy procedure,writing data of the host IO request into both the first volume and thesecond volume.
 19. The method of 18, wherein the determination that datastored in the data blocks of the first volume that are associated withthe host IO request have been copied to the second volume due to thefirst background copy procedure is determinate by using the second VBUT.20. The method of 17, wherein the step of processing one or more logicaldrive IO requests further comprising: when the host IO request is awrite request targeting the first volume and it is determined that datastored in the data blocks of the first volume that are associated withthe host IO request have not been copied to the second volume due to thefirst background copy procedure, writing data of the host IO requestinto the first volume
 21. The method of 20, wherein the determinationthat data stored in the data blocks of the first volume that areassociated with the host IO request have not been copied to the secondvolume due to the first background copy procedure is performed by usingthe second VBUT.
 22. The method of 1, further comprising: performing asecond background copy procedure for copying data blocks of the firstvolume to the third volume in the second synchronization operation,wherein a split event occurs during the second synchronization operationor when or after the second synchronization operation is completed; anddetecting whether a host IO request from a host is the first volume; andprocessing one or more logical drive IO requests associated the host IOwhen a host IO request targeting the first volume is detected.
 23. Themethod of 22, wherein the step of processing one or more logical driveIO requests further comprising: after a split event occurs, when thehost IO request is a write request targeting the first volume and it isdetermined that data stored in the data blocks of the first volume thatare associated with the host IO request have been copied to the thirdvolume due to the second background copy procedure, writing data of thehost IO request into the first volume.
 24. The method of 23, wherein thedetermination that data stored in the data blocks of the first volumethat are associated with the host IO request have been copied to thethird volume due to the second background copy procedure is determinedby using the third VBUT.
 25. The method of 22, wherein the step ofprocessing one or more logical drive IO requests further comprising:after a split event occurs, when the host IO request is a write requesttargeting the first volume and it is determined that data stored in thedata blocks of the first volume that are associated with the host IOrequest have not been copied to the third volume due to the secondbackground copy procedure, determining whether data stored in the datablocks of the first volume have been replicated to the third volumeafter the split event such that the corresponding data blocks of thefirst volume and the third volume have been synchronized, and if it isdetermined that the data blocks of the first volume have not beenreplicated to the third volume after the split event, first copying thedata stored in the data blocks of the first volume to the third volume,and then writing data of the host IO request into the first volume, andif it is determined that the data blocks of the first volume have beenreplicated to the third volume after the split event, writing data ofthe host IO request into the first volume.
 26. The method of 25, whereinthe determination that data stored in the data blocks of the firstvolume that are associated with the host IO request have not been copiedto the third volume due to the second background copy procedure isdetermined by using the third VBUT.
 27. The method of 25, wherein thestep of the determining whether data stored in the data blocks of thefirst volume have been replicated to the third volume after the splitevent such that the corresponding data blocks of the first volume andthe third volume have been synchronized is determined by using the thirdVBUT.
 28. The method of claim 14, further comprising: determining ifthere is a host IO (Input/Output) request targeting the source volume orthe destination volume for data access.